1. Field of the Invention
The present invention relates generally to the detection of ions in solution and, more particularly, to apparatus for monitoring an ionic species to determine the progress of an electrochemical titration operation.
2. Description of the Prior Art
Electrochemical titrators have long been available for analyzing chemical, biological, or biochemical substances such as blood serum. U.S. Pat. No. 4,007,105, assigned to the assignee of the present invention, describes a titrator adapted for analyzing chloride electrolyte in blood samples by coulometrically generating silver ions which combine with and precipitate the chloride as silver chloride and by amperometrically detecting the endpoint of the titration by detecting the presence of excess, uncombined silver ions. Upon detecting such uncombined ions, the coulometric generator is disabled. By measuring the coulometric current flow during a titration, and hence measuring the quantity of silver ions generated in order to precipitate the chloride, a quantitative measure of the chloride level in the blood sample is obtained.
The foregoing titrator employs a coulometric generator comprising a large silver coulometric anode and a smaller platinum cathode, and an amperometric detector comprising a pair of small silver electrodes. During titration silver ions are liberated from the coulometric anode which disintegrates over a period of time. With this in mind, the coulometric anode is made relatively large to increase its operating life and is designed to be readily replaced with a new electrode as required.
The small silver amperometric electrode pair operate to detect the presence of uncombined silver ions following precipitation of all of the sample chloride. To this end, a relatively small predetermined potential difference (e.g., 100 mv) is applied across the amperometric electrodes. During titration, the silver ions liberated from the coulometric anode combine with the sample chloride. At such time no excess uncombined silver ions are present and hence no significant current flow takes place between the amperometric electrodes. When all chloride is precipitated, further generation of silver ions by the coulometric generator introduces excess silver ions into the solution. Such excess uncombined ions enable a substantially larger current to flow between the amperometric detecting electrodes and this current flow is employed to signal the end of the titration and to initiate a turn-off of the coulometric generator.
A drawback of the foregoing amperometric detecting arrangement resides in the fact that the flow of current therebetween liberates silver ions from the amperometric anode and hence the amperometric anode slowly disintegrates by such action. Since the amperometric anode is by design relatively small, any disintegration should be minimized. This is particularly critical where all electrodes (except the large coulometric anode) are combined in a single electrode module, as taught in the foregoing patent. Obviously, the useful life of the module is limited to that of the shortest lived electrode--herein the amperometric anode.